Auto merge of #56813 - oli-obk:main_🧶, r=pnkfelix
Always run rustc in a thread
cc @ishitatsuyuki @eddyb
r? @pnkfelix
[Previously](https://github.com/rust-lang/rust/pull/48575) we moved to only producing threads when absolutely necessary. Even before we opted to only create threads in some cases, which [is unsound](https://github.com/rust-lang/rust/pull/48575#issuecomment-380635967) due to the way we use thread local storage.